1 Copyright © 2005 by ASME Proceedings of IMECE2005 2005ASME International Mechanical Engineering Congress & Exposition November 2005, Orlando, FL IMECE2005-82260 AUXETIC CELLULAR MATERIALS AND STRUCTURES Joseph N. Grima Department of Chemistry University of Malta, Msida, Malta joseph.grima@um.edu.mt Ruben Gatt Department of Chemistry University of Malta, Msida, Malta rubengatt@gmail.com Pierre-Sandre Farrugia Faculty of Science University of Malta, Msida, Malta pierresandrefarrugia@yahoo.co.uk Andrew Alderson Center for Materials Research and Innovation, University of Bolton, United Kingdom a.alderson@bolton.ac.uk Kenneth E. Evans Department of Engineering, University of Exeter, United Kingdom k.e.evans@ex.ac.uk ABSTRACT Auxetic materials and structures exhibit the very unusual property of becoming wider when stretched and narrower when squashed (i.e. they have a negative ‘Poisson’s ratio’). This property results in many beneficial effects in the characteristics of the system that make auxetics superior to conventional systems in many practical and high tech applications, including aeronautics where, for example, auxetics are being proposed as potential components for the production of better quality lifting devices such as helicopter rotor blades or airplane wings. This work reviews and discusses the behaviour of known and novel cellular systems, which exhibit this unusual but highly desirable property. Keywords: auxetic, negative Poisson's ratios, cellular, honeycombs, mechanical properties. INTRODUCTION Auxetic materials exhibit the unexpected property of becoming wider when stretched and narrower when squashed [1], that is they have a negative Poisson’s ratio (fig. 1). This unusual behaviour is not commonly observed in materials that are normally employed in everyday life. In fact, although it is has been known for a long time that that negative Poisson’s ratios could theoretically exist (the classical theory of elasticity states that it is possible for isotropic three dimensional materials to exhibit Poisson’s ratios in the range –1 ≤ ν ≤ +0.5) until very recently, the prospect of making use of such materials on a large scale was not researched. In fact, when negative Poisson’s ratios were first reported for single crystalline iron pyrites in the first half of the 20 th century, it was attributed to twinning defects and regarded as an anomaly [2]. Fig. 1: Auxetic vs. conventional behaviour However, in the late 1980’s, the study of materials exhibiting negative Poisson’s ratios became more established and since then, negative Poisson’s ratios have been predicted, discovered or deliberately introduced in several classes of naturally occurring and man-made materials including foams [3–9], polymers [1, 10–14], composites [15, 16], gels [17, 18],